This present invention enhances the production capacity from oil and gas reservoirs by combining a downhole gravitational separation process and submersible downhole pumps. In on of the preferred embodiments the downhole gravitational separating process win be preformed to a such degre of pureness that the oil will remain gas-free throughout the whole transportation of the oil, which especially is important when oil pressure is significantly dropping when transported upwards through production tubings.
Most reservoirs contains a mixture of oil and gas, but the ration may vary. It is also usual that oil contains water and other liquids to some extent, so the further use of the term oil is therefor meant to also include water and other liquids.
Production methods of such reservoirs are restricted by the well-head-pressure, that is the reservoir pressure minus the pressure drop towards the surface (pressure drop due to the oil column height). The reservoir pressure is decreasing as production take place. The production of oil, that is the volume-flow of oil, is determined by a sufficiently well-head-pressure to be economical feasible, and as a minimum this requires at least a positive well-head-pressure.
To sustain or increase the reservoir pressure, a common solution is to reinject produced gas, and inject water and chemicals into the reservoir. However, in many cases, it is not physical or economic feasible to create or sustain a sufficiently reservoir pressure by these methods. This cause for a need and a desire for other solutions, such as using submersible pumps to pump the oil up.
An additional reason why one might want to make use of downhole submersible pumps, is that this allows reservoir pressure at borehole to drop, thus a larger pressure difference throughout the reservoir is created. This means a larger flow towards the borehole and larger production.
If there is a very small ration of gas present (approximately less than 10 vol % gas) in the reservoir, regular submersible downhole motor pumps are applicable, and are therefore usually used. This prolongs the production lifetime of the reservoir, and thus increase the total volume produced from the reservoir. However, if there is a large amount of gas present, these kinds of pumps are not applicable. Such pumps only allows a limited amount of gas to be entrained therein, without developing problems which may damage the pump and generally cause unsatisfactory operations.
Creative research has resulted in different kinds of centrifugal separation systems, which separates gas from the oil-gas mixture, thus making downhole pumps applicable. An example of such separating system is patent publication NO 3000515 B1, which has rotating helical bafflers within the production tube, creating larger centrifugal forces on the oil than it does on the lighter gas. A tubing within the production tube collects this separated oil, and it is then pumped upwards. U.S. Pat. No. 5,482,117 is based on the same centrifugal principle, but makes use of stationary helical bafflers in stead of rotating helical bafflers. In addition, these patents suggest to make use of the separated gas, as so called xe2x80x9cgas-liftxe2x80x9d. Gas due to it has low weight per volume, does not have the same pressure drop in heights as oil, thus when it is injected in the oil column above the pump this will increase the pressure, lifting the oil upwards.
Systems based on centrifugal separation are characterized by requiring complex baffler constructions, and complex tubing arrangements to make space for: bafflers, pumps and the two tubes for the gas and the oil. Further, rotational bafflers are fairly energy (electricity) consuming. At some production sites electricity is not easily or economic available, such as off-shore production sites.
In addition when a very high ratio of gas occur, in the case for rotational bafflers, the baffler motor might overheat due to the reduced heat capacity in oil-gas mixture. In the case for stationary bafflers, a pressure drop will occur proportional with the number of turns the helical baffler has, thus larger pump power will be required. The centrifugal separation does not necessary imply a totally separation, thus an additional separation is needed at ground level. Further problems could be that the helical bafflers may easily be plunged up, even if small object enters into the path.
Due to these problems it is desirable to find new solutions making regular submersible downhole motor pumps applicable for reservoirs containing gas. Until now, centrifugal separation methods seems to be the only solution the industry has come up with to solve this problem.
However, to separate gas to make regular submersible downhole motor pumps applicable in reservoirs containing gas, is only one of the objectives by this present invention. During production of oil, it is transported vertically through production tube, thus the oil pressure will decrease. So, even if gas apparently seems separated from oil down in the tubing, this is not correct since gas will xe2x80x9cappearxe2x80x9d proportionally with the pressure decrease in height, due to gas is much more compressible than oil. That is, the gas together with oil is so compressed at high pressures (at large depths) that it seems not to be present. When the pressure of this mixture decreases, the gas expand (in volume) relatively much more than oil. This causes many problems. One is that the oil produced is not pure enough to be transported away and therefore needs additional separation. When oil is transported over long distances this requires additional pumping activities, which means no gas can be present. So, another accomplishment of this present invention is performing the downhole separating process to a such degree of pureness that the oil will remain gas-free throughout the whole transportation of the oil production.
This present invention enhances production capacity from oil and gas reservoirs. This is primary achieved by making regular submersible downhole motor pumps applicable in oil reservoirs containing gas in a more economical and profitable way. This could partly be achieved by making a solution not being depended on centrifugal separation. Further, this invention is understood to be easier and less expensive to make and use, and also be more sustainable. In addition this invention has the opportunity, if desired, to produce totally separated products of oil and gas.
The core idea in this present invention is to place a pump 5 in a  less than  less than bath greater than  greater than  of oil 14, in which oil  less than  less than bath greater than  greater than  14 makes a gas seal assuring the pump only to be imposed to oil without gas. In one of the preferred embodiments (FIG. 1), oil and gas from the reservoir flows trough perforations 2, into a ring-space 3. This creates a significantly pressure drop of the mixture, in which creates turbulence so the gas content will separate from the oil. The pressure drop is regulated by the gas pressure valve 4.
Mainly due to gravity and to some extent the pumping activity 5, the mixture flow is directed downwards. During a limited time this mixture will appear in a turbulent flow, then most gas will escape from the oil. The remaining gas content will be grouping together as larger bobbles. Since gas is lighter than oil, the gas contents will increase its buoyancy relative to oil during the grouping process. This increased buoyancy creates a flow of gas in the opposite direction, that is upwards, at a speed relative larger than the oil flow downwards. At some height level 11, the turbulent flow turns to laminar flow, and it is from this height level and downwards the  less than  less than bath greater than  greater than  of oil is present. Then, only an insignificant content of gas is present, this is the height level 11, which also is referred to as the gas seal 11.
Viewed isolated, this gravitational separation principle is in its self a technology which from U.S. Pat. No. 2,293,196 was known to the industry already in 1939 (see FIG. 4). And of course, at that point of time, there were also an awareness of the consequences of small well-head-pressure. The same U.S. Pat. No. 2,293,196 suggested therefore to repressure the reservoir by the produced gas, however despite this awareness, no submerged pump was then suggested to be used underneath the gas-seal. This opportunity was probably not seen by this US patent inventor, nor the remaining industry developers so far, due to the obstacles such an implementation of a submergible pump would imply.
First and most important, a regulation system that can assure that gas-seal 11 never to break underneath the intake level 15 for the pump 5, is the main key that enables this present invention to operate. A regulation according to the regulator that U.S. Pat. No. 2,293,196 suggested, can not accomplish this. Locking at FIG. 4, representing the main idea from U.S. Pat. No. 2,293,196, we can get an understanding of the restrictions the regulator 18 in that patent causes. It is also important to emphasize that this U.S. Pat. No. 2,293,196 main objective is to separate, and has no suggestions of adding lifting power what so ever to the oil column. When fluid height level 20 approaches the lower end of the inner tubing, the fluids well-head pressure (PWH) increases relatively to the gas pressure (GP), like a wave motion. The diaphragm 19 inside the regulator 21 is mechanical linked to the fluid production flow valve 21, then reducing the production allowing fluid to accumulate. Fluid level 20 then increases, reducing fluids well-head pressure (PWH) relatively to gas pressure (PG), thus making the valve 21 open for production again.
However, if a pump had been put at the end of the inner tubing at the separation system presented in FIG. 4, the regulator 18 would not work A reduction of the height level 20 would not have any significant impact on the well-head pressure (PWH), since this pressure difference caused in ring-space would have to work its way through the pump, or even maybe several pumps. Since there would be no significantly pressure variations relative to gas pressure (GP), the diaphragm 19 would not move, thus no regulation. To understand that the pumps tends to sustain the well-head pressure (WHP), it is fruitful to observe what would happen if height level 20 increases. Then the pump would continue its pumping operation, therefore causing an approximately sustained well-head pressure (PWH). And even if these pressure variations could work its way through the pump(s), it would at least cause a to large delay on making an effect on the regulator 18, that is the response time would be way to slow.
A conclusion of the above mentioned, is that the present invention requires a much more sophisticated regulation system to sustain the desired height of the gas seal, if a pump is placed into the inner tubing of the regulation system presented in FIG. 4. According to this present invention, now referring to FIG. 1, the most efficient way to regulate this is to survey the height level of the gas seal by a height measurer 9, and process this information via a regulating processor to control the regulation control variables. The regulating processor would preferably be a computer processor (not drawn). Anyhow, the key for success here is to identify the need for a surveillance of the height of the gas seal.
Secondly there are some geometrically restrictions which must be met, in order to geometrically allow gas to separate, mainly this means that the spacing in ring-space 3 need to be large enough to physically enabling gas bobbles to escape. Also, the distance from perforations 2 and gas-seal 11 must be long enough to give the gas enough time to escape.
An other example of known gravitational separation is UK Patent Application GB 2 326 895, which separates water from a oil-gas mixture over the well-tube distance, by orienting the well-tube in a non-vertical direction. Since the oil-gas mixture is lighter then water, it will position itself at the top part of the tube and is then guided into a inner tubing and then pumped further. However, the pump here has no assurance not to be imposed by significantly amounts of gas.